Conventionally, it is well known that a urea selective catalytic reduction system (urea SCR system) uses a urea to purify an exhaust gas of an internal combustion engine as a post operation. JP-2003-269142A discloses that a nitrogen oxide (NOx) included in an exhaust gas is reduced by adding the urea to the exhaust gas in the urea SCR system. In this case, the urea is mixed with water as a urea water and is injected to the exhaust gas. A urea-water injector is controlled to be open or closed according to an injection control portion. The injection control portion controls an injection amount of the urea water added to the exhaust gas, according to an amount of the NOx in the exhaust gas or an operation state of the internal combustion engine. According to JP-2003-269142A, the injection control portion detects the amount of the NOx included in the exhaust gas flowing through an exhaust passage, computes the injection amount of the urea water suitable for the amount of the NOx, and controls the urea-water injector.
However, the urea-water injector may cause variation in the injection amount of the urea water with respect to a command value from the injection control portion, due to individual differences or an age-related deterioration. When the variation in the injection amount of the urea water added to the exhaust gas is generated, a reduction of the NOx included in the exhaust gas is insufficient, and ammonia may be generated due to the urea water insufficient in the exhaust gas.